Recent years have seen active development of technology related to a radar apparatus equipped on a vehicle (hereinafter, referred to as an in-vehicle radar apparatus). As an example, a radar apparatus using a direct sequence spread spectrum scheme (hereinafter, referred to as a DS-SS radar apparatus) is proposed.
Specifically, the DS-SS radar apparatus modulates (spreads), at a transmitting side, a narrow-band signal into a broad-band signal using a spread code, and transmits, as a radar wave, the broad-band signal obtained through the modulation. At a receiving side, the DS-SS radar apparatus receives a reflected wave that is obtained when the transmitted radar wave is reflected from an object, and demodulates (despreads) the received signal, which is received using the spread code, into the initial narrow-band signal. At the time, a spread code identical to the spread code used for modulating a narrow-band signal into a broad-band signal at the transmitting side is generated at the receiving side, by shifting the spread code by a length equal to or smaller than the bit width of the code. Between the generated spread code and the received signal, a correlation operation (despreading) is performed each time. A narrow-band signal can be obtained when the shifting of the spread code, being shifted little by little, reaches the amount of displacement corresponding to the propagation time from the transmission of the radar wave until the reception of the reflected wave obtained when the transmitted radar wave is reflected from the object. The correlation operation is repeatedly performed until reaching the amount of displacement corresponding to the scan range. Here, the amount of displacement per correlation operation, that is, the bit width of the spread code determines the distance resolution of the radar apparatus. Thus, the narrower the bit width of the code is, that is, the higher the chip rate of the code is, the radar apparatus is considered as having higher resolution.
In addition, an in-vehicle radar apparatus is used in the detection of a vehicle ahead and a rear obstacle for purposes such as: safety improvements including collision avoidance, improvement of conveniences as represented by vehicle reversing aids, and improvements in drivability such as automated cruise. In such purposes, it is necessary to suppress the influences of unnecessary radio waves, such as interference by electromagnetic waves emitted by a radar apparatus of the same kind that is equipped on a vehicle other than the current vehicle.
For this reason, for a spread code used for the DS-SS radar apparatus, it is preferable that the code should have excellent cross-correlation property which enables a vehicle to avoid interference with the radar wave from an apparatus of the same kind equipped on another vehicle, and should also have excellent autocorrelation property which enables the vehicle to avoid interference with a radar wave transmitted from the vehicle. In addition, it is preferable that the DS-SS radar apparatus should have a function that allows conversion into an arbitrary spread code when interfered by a radar apparatus using the same spread code and equipped on another vehicle.
Additionally, when despreading is performed at the receiving side, it is necessary to generate, with respect to the spread code, a delay corresponding to the propagation time of the radar wave so as to correlate the spread code and the delay; therefore, a code generation apparatus capable of generating an arbitrary delay amount (delay time) is required. Furthermore, in the case where the radar apparatus is applied as a short-range radar, a distance resolution of a few centimeters is required, thus necessitating code generation at a high-speed chip rate.
Thus, generally, a pseudo-noise code (hereinafter, referred to as a PN code) having a common rule for both transmitting and receiving sides is used for the spread code. The representative code includes an M-sequence code and a Gold-sequence code.
FIGS. 1 and 2 are diagrams showing the structure of a PN code generator in conventional technology. As FIG. 1 shows, the PN code generator 12 includes a shift register 11 and an exclusive OR operation circuit (EX-OR) 13. Here, as an example, the shift register 11 is assumed as an n-staged shift resister. Then, the exclusive OR operation circuit (EX-OR) 13 performs exclusive OR operation on the logical values of the last stage and a mid stage of the shift register 11, so as to generate a PN code while causing the obtained logical value to be inputted into the initial stage. However, for the PN code generator 12 including the shift register 11, it is difficult to change the tap location at which to extract the logical value of the mid stage, and therefore it is difficult to change the PN code upon request.
In contrast, as FIG. 2 shows, the PN code generator 23 includes: a flash memory 23b; a write controller 23c for writing a code into the flash memory 23b; a read controller 23d that reads the code; and a micro processor unit (MPU) 23a that outputs the code from a designated address. The PN code generator 23 can generate a code having an arbitrary delay amount by generating an arbitrary code and designating a readout address (See, for example, Patent Reference 1).
Patent Reference 1: Japanese Unexamined Patent Application Publication No. H07-86984.